Replace platform-specific GetMilliseconds using std::chrono.

src/platform/cocoamain.mm

@@ -70,17 +70,6 @@ std::string CnfThawString(const std::string &val, const std::string &key) {
 
 /* Timer */
 
-int64_t SolveSpace::GetMilliseconds(void) {
-    clock_serv_t cclock;
-    mach_timespec_t mts;
-
-    host_get_clock_service(mach_host_self(), SYSTEM_CLOCK, &cclock);
-    clock_get_time(cclock, &mts);
-    mach_port_deallocate(mach_task_self(), cclock);
-
-    return mts.tv_sec * 1000 + mts.tv_nsec / 1000000;
-}
-
 @interface DeferredHandler : NSObject
 + (void) runLater:(id)dummy;
 + (void) runCallback;

src/platform/gtkmain.cpp

@@ -194,12 +194,6 @@ static void CnfThawWindowPos(Gtk::Window *win, const std::string &key) {
 
 /* Timers */
 
-int64_t GetMilliseconds(void) {
-    struct timespec ts;
-    clock_gettime(CLOCK_MONOTONIC, &ts);
-    return 1000 * (uint64_t) ts.tv_sec + ts.tv_nsec / 1000000;
-}
-
 static bool TimerCallback() {
     SS.GW.TimerCallback();
     SS.TW.TimerCallback();

src/platform/w32main.cpp

@@ -772,15 +772,6 @@ bool SolveSpace::FullScreenIsActive()
     return false;
 }
 
-int64_t SolveSpace::GetMilliseconds()
-{
-    LARGE_INTEGER t, f;
-    QueryPerformanceCounter(&t);
-    QueryPerformanceFrequency(&f);
-    LONGLONG d = t.QuadPart/(f.QuadPart/1000);
-    return (int64_t)d;
-}
-
 void SolveSpace::InvalidateText()
 {
     InvalidateRect(TextWnd, NULL, false);

src/solvespace.h

@@ -24,6 +24,7 @@
 #include <unordered_map>
 #include <map>
 #include <set>
+#include <chrono>
 #ifdef WIN32
 #   include <windows.h> // required by GL headers
 #endif

src/util.cpp

@@ -61,6 +61,12 @@ bool SolveSpace::FilenameHasExtension(const std::string &str, const char *ext)
     return true;
 }
 
+int64_t SolveSpace::GetMilliseconds()
+{
+    auto timestamp = std::chrono::steady_clock::now().time_since_epoch();
+    return std::chrono::duration_cast<std::chrono::milliseconds>(timestamp).count();
+}
+
 void SolveSpace::MakeMatrix(double *mat,
                             double a11, double a12, double a13, double a14,
                             double a21, double a22, double a23, double a24,